1. Field of the Invention
The present invention relates to a compact zoom lens system, and more particularly to a compact zoom lens system with a shortest focal length of about 50 mm to 135 mm and with a zoom ratio of about 2 to 3.
2. Description of the Prior Art
As a mechanical compensation type zoom lens system which has a same shortest focal length and a same zoom ratio as that of the present invention, the following three types have been proposed conventionally:
(A) 4-unit type telephoto zoom lens system, PA1 (B) 2-unit type standard zoom lens system, PA1 (C) 3-(or 4-) unit type standard zoom lens system with a large zoom ratio in which the first lens unit located at the most object side is shiftable in a zooming operation.
The 4-unit type telephoto zoom lens system in category (A) includes, from the object side, a focusing lens unit (I) of a positive refractive power, a variator lens unit (II) of a negative refractive power, a compensator lens unit (III) of a positive or a negative refractive power, and a master lens unit (IV) of a positive refractive power, as shown in FIG. 1. The variator lens unit (II) is shifted for a zooming operation, and the compensator lens unit (III) is simultaneously shifted for compensating the change in the image plane due to the shifting of the variator lens unit (II).
In this type of lens system, the following defects are presented;
1. The refractive power of the variator lens unit (II) is relatively strong, since the zooming operation is achieved only by shifting of the variator lens unit (II). Thus, it is difficult to make the variator lens unit simple in construction. On the contrary, if the refractive power of the variator lens unit (II) becomes weak, the total length of the whole lens system, i.e., the distance from the object side surface of a lens element located at the most object side to the image plane, must be extended since the shifting distance of the variator lens unit (II) in a zooming operation becomes large.
2. An inverted telephoto type lens system is formed in the shortest focal length condition of the zoom lens system as shown in FIG. 1 since the negative refractive power of the variator lens unit (II) is relatively strong. This means that the total length of the whole lens system becomes long in comparison with the focal length thereof.
3. It is difficult to make each of the lens units simple in construction, since each lens units, especially the variator lens unit (II) and the master lens unit (IV), have relatively strong power.
Meanwhile, the 2-unit type standard zoom lens system described in the above category (B), as shown in FIG. 2, includes, from the object side, the first lens unit (I) of a negative refractive power and the second lens unit (II) of a positive refractive power. The first lens unit (I) and the second lens unit (II) are simultaneously shifted along the optical axis to achieve a zooming operation and to compensate the change in the position of the image plane. In this type of a zoom lens system, the following drawbacks are presented;
1. As understood from FIG. 2, a strong inverted telephoto type is formed at the shortest focal length (S) and also an inverted telephoto type is formed through the whole zooming range. Accordingly, if the focal length range is set in the telephoto region, the total length of the whole lens system becomes long.
2. If the refractive power of each unit is increased for more compactness of the lens system, the number of the lens elements of each unit will be also increased. Accordingly, it is impossible to make the construction simple. Conversely, if the refractive power is decreased, the shifting distance of each group for the zooming operation and for the focusing operation will unfavorably become large.
The 3-unit standard zoom lens system in the above category (C), as shown in FIG. 3, includes, from the object side, a first lens unit (I) of a positive refractive power, a second lens unit (II) of a negative refractive power and a third lens unit (III) of a positive refractive power. The first lens unit (I) and the third lens unit (III) are simultaneously shifted along an optical axis in the zooming operation to share the zooming effect into both lens units so that a high magnification ratio can be obtained. However, if this type lens system is applied to a telephoto zooming lens system, the following defects are presented;
1. Although the total length of the lens system can be shorter than that of the 4-unit zoom lens system in category (A), the lens system forms an inverted telephoto type at the shortest focal length (S). Accordingly, the desired compactness can not be achieved.
2. Since the lateral magnification of the second lens unit (II) and that of the third lens unit (III) are negative, it becomes difficult to decrease the number of lens elements of each group without weakening the refractive power of each of the second and third lens units (II) (III). On the contrary, the desirable compactness can not be obtained if weakening the refractive power of each of the second and third lens units (II) (III) is not permitted.